The present invention relates to force balanced instruments of the closed loop type, and more particularly concerns control of a force balanced instrument having a linear output proportional to sensed input and in which a processed feedback signal provides a forcing signal to a selected side of the sensitive element.
Force balanced sensing instruments often use a sensing member that is movable from a nominal position in response to an input condition that is to be sensed. A position pickoff provides an indication of sensing member position and a feedback signal, based on the pickoff signal, applies a force to the sensing member that tends to return it to its nominal position The feedback signal may provide an instrument output signal representing the sensed input condition. It is generally desired that the instrument output signal be proportional to the input condition, and thus in many types of electrostatic and electromagnetic force balance sensing instruments special techniques are provided or required to obtain a linear relation between the instrument output and the sensed input. In electrostatic and electromagnetic instruments the forces applied by the instrument forcer are not linearly related to the feedback voltage or current supplied to the forcer. Furthermore, for optimum operation of the instrument itself, it is preferred that the feedback force applied by the feedback control network have a linear relation to the sensed input. Thus special techniques have been employed for obtaining such linearity.
For example, in an electrostatic force balanced accelerometer of the type shown in U.S. Pat. No. 4,679,434, for Integrated Force Balanced Accelerometer of Robert E. Stewart, electrostatic forcing in a closed loop system is employed to position and obtain an output from a pendulous inertial mass. The electrostatic forcing system employs a capacitative pickoff electrode on each side of a pendulous member that has been etched from a silicon substrate. The electrodes also apply nominally equal and opposite bias forces to the pendulous member, to which is applied a control voltage. In another control arrangement for an accelerometer of this type, a fixed bias voltage, V, is applied to the pendulous inertial mass, and feedback voltages +v and -v, are applied concurrently to pickoff and forcing electrodes on opposite sides of the mass. Accordingly, omitting factors such as gap variation, parallelism, dielectric constants and the like, which may also affect force, the force applied by each electrode to the pendulous mass is proportional to (V+v).sup.2 and (V-v).sup.2, respectively. The net force on the pendulum applied by this control system then is the difference between the two forces, which is effectively proportional to 4 vV. As the bias voltage V is a constant, the feedback voltage, v, is proportional to the feedback force applied, and is also linearly related to the input acceleration experienced by the pendulous inertial mass.
This system has a number of problems, including the large negative spring effect associated with the required bias electrical fields. Even in the absence of any input acceleration to be sensed, the bias fields are required, and, since the bias field may vary and the pickoff null position may vary, the instrument may have poor null stability and repeatability. In the described prior system, many factors, such as gap variation, aging of components, temperature variations and the like provide sources of error that may result in a spurious output and decreased null stability. Potentially this spurious bias error is a large error. Small variations in electric field are exacerbated by the large negative spring effect in voltage biased systems. This negative spring effect is caused by the two large bias fields and may be quite large for typical ranges of accelerometer inputs
The problems described above that exist in this oppositely biased electrostatically forced silicon accelerometer may also exist in other types of electrostatic or electromagnetic force balanced instruments where the applied feedback force is not linearly related to the feedback signal.
Accordingly, it is an object of the present invention to provide a closed loop force balanced instrument that avoids or minimizes above-mentioned problems.